Dichroic Parametric Mirror Techniques

This report summarizes the results of a twelve-month program whose goal was to develop mirrors which are suitable for use with optical parametric oscillators having outputs in the 1.5 to 4.5 micron region. The properties which such mirrors must have are 1 low scatter 2 the correct spectral reflectances at signal, idler, and pump wavelengths and 3 high resistance to optical damage. The mirrors which were studied on this program were designed for use with a lithium niobate parametric oscillator which is pumped by 1.08 microns. Several different types of multilayer coatings were fabricated and tested. These were ZnS-ThF4, ZnS-ThF4 overcoated with Al2O3, ZnS-cryolite, Zns- cryolite overcoated with Al2O3, ZnS-ThF4 undercoated and overcoated with Al2O3, and a hard coating of unknown composition. The tests performed on each coating included 1 microscopic inspection to determine scatter properties, 2 spectrophotometer traces to determine spectral transmittance, 3 1.08 micron damage threshold tests, and 4 operation of an optical parametric oscillator using the coatings. Best overall results were obtained with mirrors composed of ZnS-ThF4 undercoated and overcoated with Al2O3. Measurements were also performed to determine the surface-damage thresholds of uncoated and coated lithium niobate it was found both soft e.g. ZnS, ThF4, cryolite, etc. and hard e.g. Al2O3 coatings raise this threshold, with the greatest improvement effected by the Al2O3 coating.